Organic electroluminescent devices (hereinafter, simply referred to as an “organic EL device” or “organic EL element”) have been extensively developed in expectation of their use as solid-state luminescent devices or light emitting devices for use in inexpensive large full-color displays.
In general, such an organic EL device has a structure in which a light emitting layer is provided between a cathode and an anode. When an electric field is applied between the cathode and the anode, electrons are injected into the light emitting layer from the cathode side, and holes are injected into the light emitting layer from the anode side.
The injected electrons and holes are recombined in the light emitting layer, which then causes their energy level to return from the conduction band to the valence band. At this time, excitation energy is released as light energy so that the light emitting layer emits light.
In such organic EL devices, it has been known that a layered device structure, in which organic layers formed of organic materials having different carrier transport properties for electrons or holes are provided between a light emitting layer and a cathode and/or an anode, is effective in obtaining a high-efficiency organic EL device with high luminance (see JP-A No. H09-255774).
Therefore, in the field of an organic EL device having a structure in which a light emitting layer and a carrier transport layer are laminated between an anode and a cathode, various researches are carried out on the issues such as molecular structures of a light emitting material and a carrier transport material as well as an appropriate number of the lamination of a light emitting layer and a carrier transport layer and positions and shapes of these layers for the purpose of obtaining improved characteristics such as light emitting efficiency and the like.